Juan Lacruz, La Caņada Observatory, May 2006, version draft
According to the latest views on cometary populations, comets originate in three different reservoirs, the Oort cloud, the Kuiper belt and the main asteroid belt.
The Oort cloud, at 3.000-50.000 AU from the Sun, supplies the Halley family (HFC) and long period comets, The Kuiper belt, beyond Neptune, at a range of 30-50 AU from the Sun, supplies the Jupiter family comets (JFC), recently Henry H. Hsieh* and David Jewitt (1) have found what seems to be a third family of comets originating in the asteroid main belt (MBC).
The Halley and Jupiter families are well stablished, and have have numerous members, but the MBC family only has the following members so far :
|Asteroidal designation||Cometary designation||a (AU)||e||i||H||Tj|
|(7968) Elst-Pizarro||133P Elst-Pizarro||3.16||0.16||1.39||14||3.18|
|-||P/2005 U1 READ||3.16||0.25||1.27||n/a||3.15|
|(118401) 1999 RE70||-||3.20||0.19||0.24||15.1||3.17|
Dynamically, two of the three MBCs (EP and 118401) are associated with the Themis collisional family, with P/Read falling just outside this family due to its slightly high eccentricity. This family association is possibly an artifact of observational selection and there may well be other MBCs not in this family.
Thermal averaging inside the nuclei will lead to deep interior temperatures near the local blackbody value. At 3.2 AU (the approximate semimajor axis distance of all three MBCs), this temperature is 155 K, at which ice is thermodynamically stable. At smaller distances, higher deep interior temperatures may prevent the survival of ice over the age of the Solar system.
Careful observations in search of comae on closer asteroids are needed, it is possible that non-Themis objects that outgas also exist but have so far escaped detection (1)
I've written an interactive planner for the observation of objects of the Themis family, be aware that the data is pulled from the MPCORB and thus the elements are osculating and not proper, this may lead to the inclusion of some outliers in the plan.
(1) A Population of Comets in the Main Asteroid Belt, 19 January 2006; accepted 27 February 2006, Published online 23 March 2006; 10.1126/science.1125150
Main Belt Comets
Active Asteroids : Mistery in the Main Belt